Convectively cooled, single stage, fully premixed controllable fuel/air combustor with tangential admission

ABSTRACT

An annular combustor system is disclosed in use with a radial turbine-type gas turbine gas generator, the system having an annular housing defining a single stage combustor, an external fuel/air premixer system having a mixing chamber and a fuel valve under the control of a controller, to provide a preselected lean fuel/air ratio mixture for introduction to the combustion zone of the annular housing. A separate compressed air valve under the control of the controller can optionally be included. Compressed air conduits are used to channel a portion of the total compressed air flow to the premixer and the remainder to the dilution zone of the combustor, and a fuel conduit is used to deliver all of the fuel to the premixer. Convection cooling of the annular housing is accomplished using compressed air without diluting the fuel air ratio in the combustion zone. The premixer includes a venturi, and a fuel nozzle for spraying fuel into the venturi inlet along the venturi axis, the venturi axis being aligned substantially tangentially to the annular housing axis to provide a swirling admission of the fully premixed fuel/air mixture to the combustion zone. The venturi can include a flow-smoothing member and be heated to provide augmented vaporization of liquid fuels. The compressed air valve can be integrated with the premixer housing.

This application is a continuation-in-part of application Ser. No.08/261,256, filed Jun. 14, 1994, (U.S. Pat. No. 5,481,866) which is acontinuation of Ser. No. 08/086,833, filed Jul. 7, 1993 (abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a combustion system for gas turbine gasgenerators, gas turbine engines, or other heat devices, which canproduce low levels of oxides of nitrogen and carbon monoxide.Specifically, the present invention significantly lowers NO_(x) and COlevels by providing a nearly relates to a system, process, and apparatusfor combusting fuel in a gas turbine or gas generator module whichconstant fuel/air ratio in the combustion zone at all engine operatingconditions in addition to thoroughly pre-mixing the fuel and air priorto combustion and, if necessary, completely vaporizing a liquid fuel.

2. Description of the Art

Although gas turbine devices such as engines and gas generators do notproduce the majority of the nitrogen oxide emissions released into theearth's atmosphere, reducing those emissions will reduce the total and,in that regard, many countries have enacted laws limiting the amountsthat may be released. The reaction of nitrogen and oxygen in the air toform nitrogen oxides, like almost all chemical reactions, proceedsfaster at higher temperatures. One way to limit the amount of NO_(x)formed is to limit the temperature of the reaction. The NO_(x) producedin gas turbine devices is produced in the combustion process where thehighest temperature in the cycle normally exists. Therefore, one way tolimit the amount of NO_(x) produced is to limit the combustiontemperature.

Various attempts have been made to limit the combustion temperature andthereby NO_(x) production in both "single stage" combustors (i.e., thosehaving only a single combustion zone where fuel and air are introduced)and "multistage" combustors, including pilot burners where several,serial connected combustion zones having separate fuel and airintroduction means are used. U.S. Pat. Nos. 4,994,149, 4,297,842, and4,255,927 disclose single stage gas turbine combustors where the flow ofcompressed air to the combustion zone and the dilution zone of anannular combustor are controlled to decrease the concentration of NO_(x)in the turbine exhaust gases. In the above combustors, essentiallyunmixed fuel and air are separately admitted to the combustor, withmixing and combustion consequently occurring within the same chamber.See also Japanese Laid-Open No. 55-45739. U.S. Pat. Nos. 5,069,029,4,898,001, 4,829,764, and 4,766,721 disclose two stage combustors. Seealso German Gebrauchsmuster, 99215856.0. Again, however, fuel and airare provided to each stage at least partially unmixed with completemixing occurring within the respective combustion zones.

Attempts also have been made to utilize separate premixer chambers toprovide a premixed fuel-air flow to a combustor. Japan Laid-OpenApplication No. 57-41524 discloses a combustor system which appears topremix only a portion of the total fuel flow to a multistage can-typecombustor in a separate mixing chamber prior to introduction to thestaged combustion chambers. In U.S. Pat. No. 5,016,443, a large numberof separate fuel nozzles is used to inject fuel into an annular premixerchamber. However, the complexity of the above constructions employingmultiple fuel nozzles and fuel splitting devices can lead to controldifficulties, as well as a high initial cost.

SUMMARY OF THE INVENTION

Thus, it is an object of the invention to provide a combustor apparatusfor use with gas turbine gas generator and engine modules, whichapparatus results in low emissions of both NO_(x) and unburned fuel andfuel by-products over the entire operating range of the gas turbine gasgenerator or engine module.

It is a further object of the present invention to provide an apparatusthat is relatively less complex than other state of the art annularcombustor apparatus and systems thereby facilitating ease of operation,lower initial cost of the apparatus, and substantially improved fuel/aircontrol by the avoidance of matching a large number of separatepremixers.

In accordance with the present invention as embodied and broadlydescribed herein, the combustor system for operation with a source ofcompressed air and a source of fuel comprises a cylindrical housingdefining a single stage combustion chamber, the housing having an axisand having at least one inlet port adjacent one chamber end, the portionof the chamber adjacent said one chamber end comprising a combustionzone. The housing also has an exhaust port at the opposite axial chamberend, the portion of said chamber adjacent the opposite axial endcomprising a dilution zone, the housing also having aperture means intothe dilution zone. The combustor system also includes at least onefuel/air premixer disposed outside the housing and having means forreceiving compressed air, means for receiving fuel, and premixer chambermeans for mixing the received compressed air and fuel and delivering themixture to the combustion zone through the inlet port. The premixerchamber means includes a venturi having an inlet, an outlet, and a flowaxis. The venturi inlet is flow connected to the compressed airreceiving means and the fuel receiving means, and the outlet isconnected to the housing inlet port to deliver the fuel/air mixture tothe combustion zone. The venturi is disposed with the flow axis alignedin a substantially tangential direction with respect to the cylindricalhousing axis. The combustion system also includes first conduit meansinterconnecting the source of compressed air and the premixer and, withthe premixer compressed air receiving means, establishing a compressedair flow path for delivering a portion of the compressed air to thepremixer, and for delivering essentially the remaining portion of thecompressed air to the aperture means. The combustor system furtherincludes second conduit means interconnecting the fuel supply and thepremixer fuel receiving means and, together with the premixer fuelreceiving means, establishing a flow path for all the fuel to thepremixer. The combustor system still further includes fuel valve meansdisposed in the fuel flow path for determining the fuel flow ratetherein.

It is preferred that the venturi include means for heating the venturito augment vaporization of liquid fuels, particularly during start-up.

It is also preferred that the combustor system include an ignitorpositioned on the cylindrical housing adjacent the intersection of theventuri flow axis.

It is also preferred that a gas generator module made in accordance withthe present invention have the above-described combustor system andfurther include a mechanically independent spool with an air compressoroperatively connected to the first conduit means for supplyingcompressed air, a turbine operatively connected for receiving theexhaust gases from the exhaust port, and shaft means interconnecting theair compressor and turbine, the speeds of the turbine and air compressorbeing dependent on the fuel flow rate, whereby a substantially constantfuel/air ratio is maintained over substantially the entire operatingrange of the gas generator module.

It is still further preferred that a gas turbine engine module mode inaccordance with the present invention have the above-described gasgenerator module and further include power-producing means operativelyconnected to receive exhaust gas discharged from the turbine. Anespecially preferred power-producing means is a free-power turbine.

The present invention as embodied and broadly described herein alsoprovides a process for operating a gas turbine gas generator module tominimize NO_(X) and unburned fuel and fuel by-products, the gas turbinegas generator module of the type having an cylindrical housing with anaxis and defining a combustion chamber and also having a mechanicallyindependent spool including an air compressor, a turbine, and shaftmeans interconnecting the air compressor and turbine. The housing has atleast one inlet port proximate one axial end, the chamber portionadjacent the one axial end comprising a combustion zone, and an exhaustport and aperture means proximate the other axial end, the chamberportion adjacent the other axial end comprising a dilution zone. Theprocess of the present invention comprises the steps of supplyingcompressed air flow and fuel flow to the vicinity of the housing;continuously premixing the entirety of the fuel flow with a portion ofthe compressed air flow outside the housing and injecting the resultingfuel/air mixture into the combustion zone through the housing inlet portin a direction and with a velocity to provide swirling combustion aboutthe housing axis; admitting the remainder of the compressed air flow tothe dilution zone through the aperture means; and controlling fuel flowrate in accordance with the desired power level to provide a mixturewith an essentially predetermined lean fuel/air ratio over substantiallythe entire operating range of the gas turbine engine module.

The technical considerations for the above-described invention involvethe dynamics of the combustion process. The amount of nitrogen oxide inthe exhaust can be expressed by the following equation: ##EQU1## where Tis the flame temperature in degrees Kelvin, NO_(x) is the nitrogen oxideconcentration coming out of the combustion zone, expressed as NO₂, inparts per million by volume, and P is the pressure in atmospheres. Theflame temperature is a function of both the fuel/air ratio forcompletely premixed mixtures and of the combustor inlet air and fueltemperatures. Experience has shown that the flame in a combustor willcontinue to burn if the flame temperature is above about 2550 degreesRankine (1417 degrees Kelvin) for pure fuels, or slightly higher forfuels containing a noncombustible diluent, such as nitrogen. However, atthis level, the flame is close to extinction and the carbon monoxideemissions are high.

To have acceptably low levels of both pollutants, it is desirable toestablish a fuel/air ratio that, in conjunction with the combustor inlettemperatures, will produce a flame temperature of about 2800 to 3000degrees Rankine (1556 to 1667 degrees Kelvin). Use of the equation willshow that the NO_(x) levels will be between 0.8 and 2.0 ppmv (parts permillion by volume) at one atmosphere before the dilution air is added toreduce them still more. Experience also has shown that carbon monoxidelevels at these temperatures will be below 20 ppmv and will be evenlower at higher pressures.

The constant fuel/air ratio in the combustion chamber of the presentinvention is produced by adjusting the air flow to the premixer to beproportional to the fuel flow. Experience has shown that it is notenough to just limit the average temperature because, when a fuel isburned as drops of liquid or a diffusion gas flame, the combustionproceeds at near the stoichiometric value and the local temperature isvery high, thus producing excessive NO_(x). To produce the lowestpossible NO_(x), the annular combustor of the present inventionthoroughly pre-mixes all the fuel and combustion air in a venturichamber separate from the combustion chamber itself, and if a liquidfuel is used, evaporates the fuel before premixing the fuel and air tobe used in the combustion. Some gas turbine engine applications exhibita nearly constant air flow regardless of power level (primarily singleshaft direct-coupled electricity producers which must run at constantspeed) and some have an air flow that decreases as the power level isreduced (such as free turbine units and propulsion units). To maintain aconstant fuel/air ratio in both types of units, it is often necessary toprovide an air valve, coupled to the fuel valve, which provides theamount of air needed for a nearly constant fuel/air ratio. Obviously thevalves will be different in the two types of engines, but the principleis the same.

However, certain aspects of the present invention are highly useful evenin applications where precise control of the fuel/air ratio afforded bya separate compressed air valve is not needed, as in free-turbine andfree-jet propulsion applications. In these applications, gross controlof the compressed air is accomplished automatically by virtue of thedependency of gas generator RPM on fuel flow. Thus, the increasedcombustion efficiencies and simplicity of construction resulting fromthe tangential admission of the premixed fuel/air, and the use ofsubstantially all the compressed air flow portion not premixed with thefuel for convective cooling, are significant advantages available forsuch applications in which a compressed air valve, if included, could bepreset to a constant opening or the valve eliminated entirely.

In this invention only one combustion zone is used and the fuel/airratio and flame temperature will always be high enough to effectivelyburn the carbon monoxide and hydrocarbons. Therefore, this invention notonly produces low emissions of nitrogen oxides, but low emissions ofcarbon monoxide and unburned hydrocarbons as well by avoiding transitionzones between stages of combustion. Since this invention has only onecombustion zone, it is not necessary to separate a primary and secondarycombustion zone (multistage combustor) or to cool such a separation.Also, it may not be necessary to use a pilot flame or associatedapparatus. Furthermore, the control system is vastly simplified byhaving one fuel control valve which must be precise and, at most, onecompressed air control valve which is more forgiving where accuracy andleakage are concerned. Additional simplification is possible in certainapplications which allow the elimination of the compressed air controlvalve.

The air-fuel mixing devices particularly described and shown in detailhereinafter, provides a nearly uniform fuel/air weight ratio at itsexit. Of course, it is necessary to keep the axial velocity above theturbulent flame speed at all points within the venturi and to preventany recirculation within the fuel/air mixing system. If theserequirements are met, combustion cannot occur before the fuel/airmixture leaves the premixing device.

Other objects and advantages of the invention will be set forth in partin the description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitutespart of the specification, illustrate preferred embodiments of theinvention and, together with the description serve to explain theprinciples of the invention.

In the drawings:

FIG. 1A is a cross sectional schematic of a gas turbine engine moduleutilizing a combustor system made in accordance with the presentinvention;

FIG. 1B is a schematic end view of the apparatus shown in FIG. 1A takenin the direction AA in FIG. 1A;

FIG. 2 is a schematic cross section of a gas turbine engine module withan alternative version of the combustor system shown in FIG. 1A;

FIGS. 3A-3C are detailed cross sectional views of a test version of thepreferred fuel/air premixer component of the apparatus shown in FIG. 1A;

FIG. 4 is a detailed cross sectional view of an engine version variationof the fuel/air premixer shown in FIGS. 3A-3C;

FIGS. 5A and 5B are a cross-sectional schematic of another gas turbineengine module utilizing a combustor system made in accordance with thepresent invention;

FIG. 6 is a schematic cross section of an alternative premixerconstruction without an integrated compressed air flow valve, for use inthe gas turbine engine module shown in FIG. 5; and

FIG. 7 is a schematic cross-section of yet another gas turbine enginemodule made in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the present preferred embodiment of theinvention which is illustrated in the accompanying drawings.

With initial reference to FIG. 1A, there is shown a combustor systemmade in accordance with the present invention and designated generallyby the numeral 10. System 10 is depicted as being used in conjunctionwith radial gas turbine engine module 12. Gas turbine engine module 12includes a pressure housing 14 within which is mounted shaft 16rotatable about axis 18. Mounted on one end of a shaft 16 is radialturbine 20 for driving centrifugal compressor 22 mounted at the opposedend of shaft 16. In the configuration depicted in FIG. 1A, gas turbineengine module 12 power is taken out through a mechanical couplingarrangement shown generally at 24 adjacent centrifugal compressor 22.However, the combustor system of the present invention can be utilizedin a gas generator in association e.g., with a "free power turbine" (seeFIG. 5A), a "free-jet" propulsion unit (not shown), or any other turbineengine system version as one skilled in the art would immediatelyrealize. Also, the present invention is not limited to use in a radialgas turbine engine or gas generator module but, at least in its broadestextent, could be used with axial or mixed axial-radial turbine engineand gas generator modules as well.

With continued reference to FIG. 1A, gas turbine engine module 12operates generally as follows. Air enters centrifugal compressor 22 in adirection designated by the arrows 26, is centrifugally accelerated toincrease its velocity, whereupon it enters diffuser 28 to increasestatic pressure. The compressed air exiting diffuser 28 is collected ina plenum chamber 30. Thereafter, compressed air from plenum 30 is mixedwith fuel from a fuel source 32 by means of premixer 60 of combustorsystem 10, to be described in more detail hereinafter, to produce hotexhaust gases which flow past inlet guide vanes 34 to radial turbine 20,where power is extracted. The exhaust gases from turbine 20 are ductedto the atmosphere or to a subsequent engine module. In the case of freepower turbine arrangement, the gases exiting turbine 20 would be ductedto the free power turbine for extraction of further power.

In accordance with the present invention, the combustor system includesa cylindrical housing defining a combustion chamber, the housing havingan axis and having at least one inlet port adjacent one axial chamberend. Importantly, the portion of the chamber adjacent the one axialchamber end comprises a single stage combustion zone. An exhaust ispositioned at the opposite axial chamber end, with the portion of thecombustion chamber adjacent the opposite axial chamber end comprising adilution zone. The housing further has aperture means in the form ofdilution ports in flow communication with the dilution zone.

As embodied herein, and with continued reference to FIG. 1A, combustorsystem 10 includes annular combustor lining housing 40 which isgenerally toroidal in shape. Although the preferred embodiment isillustrated with an annular housing, a "can-type" cylindrical housingcould also be used. Housing 40 is contained within pressure vessel 14and defines an axis 42 essentially coincident with gas turbine enginemodule axis 18. Housing 40 is closed at axial end 44 except for inletport 43, but is open at axial end 46 to form an annular exhaust port 48.Exhaust port 48 is in flow communication with radial turbine 20 throughchannel 50 past inlet guide vanes 34.

With continued reference to FIG. 1A, toroidal chamber 52 defined byhousing 40 comprises two generally axial sections with differentfunctions. Section 54 adjacent axial end 44 comprises a single stagecombustion zone and section 56 adjacent housing end 46, comprises adilution zone. A plurality of apertures 58a, 58b are provided in housing40 opening into dilution zone 56. Dilution ports 58a are a series ofapertures formed in the outer peripheral surface of housing 40, whiledilution ports 58b are a series of apertures formed in an innerperipheral surface of housing 40, relative to housing axis 42. Theaperture means generally comprising dilution ports 58a, 58b provide forthe introduction of compressed air into the dilution zone 56 ofcombustion chamber 52 from compressed air conduit means which will bedescribed in more detail hereinafter. However, dilution apertures neednot be placed in both inner and outer walls of the combustion liner. Forexample, aperture 58a may be eliminated if apertures 58b are used andsized to accommodate the entire dilution flow rate.

Further in accordance with the present invention, at least one fuel/airpremixer disposed outside the cylindrical housing is provided for mixinga portion of the compressed air flow with fuel to provide a fuel/airmixture and delivering the mixture to the combustion zone through theinlet port. The fuel/air premixer includes means for receiving thecompressed air, means for receiving the fuel and also chamber means forflow-smoothing the received compressed air and for mixing the receivedcompressed air and fuel. As embodied herein and with continued referenceto FIG. 1A, combustion system 10 further includes a single fuel/airpremixer designated generally by the numeral 60. Premixer 60 includeshousing assembly 62 for receiving the compressed air from conduit meanswhich will be described in more detail hereinafter, and a single fuelnozzle 64 for receiving fuel from fuel source 32 via fuel line 66. Fuelnozzle 64 depicted in FIG. 1A is an "air-blast" type fuel nozzleespecially advantageous for use with liquid fuel to provide atomizationand thus enhance vaporization. However, use of an "air blast" nozzlewith gaseous fuel can provide benefits in terms of providing an initialmixing of the fuel with air prior to admission to the venturi elementwhich will be described hereinafter. Therefore, the combustion system ofthe present invention is not restricted to the use of liquid fuel or an"air-blast" fuel nozzle, but gaseous fuel and other types of fuelnozzles, such as swirling-type nozzles, can be used as well.

Fuel/air premixer 60 further includes mixing chamber means in the formof venturi 68 having venturi inlet 70 disposed within fuel/air premixerhousing assembly 62 and venturi exit 72 connected to inlet port 43.Generally, venturi exit 72 should terminate flush with inlet port 43 andnot protrude appreciably into combustion zone 54 for reasons that willbe discussed hereafter. Venturi 68 defines a flow axis 74, and fuelnozzle 64 is positioned to deliver a fuel spray into venturi inlet 70substantially along axis 74. The cross sectional flow area anddimensions of venturi 68 are chosen to provide vigorous and completemixing of the fuel and compressed air within the venturi chamber and adirected flow of the resulting mixture along venturi axis 74 tocombustion zone 54, such as indicated schematically by arrow 76. Theflow area of venturi exit 72 should be chosen such that minimumvelocities of the mixture (i.e., during idle) are greater than the flamepropagation speed of the fuel/air mixture. Flame holder means such asdepicted schematically as 78 may be provided proximate venturi exit 72to enhance the stability of combustion in combustion zone 54.

It has been found that flame holders such as 78 may not be needed, asthe high degree of mixing (and vaporization of liquid fuels) in theventuri tend to provide stable combustion in the low velocity annularchamber defining combustion zone 54. In such applications, thesubstantial absence from the combustion zone 54 of internal structuralappendages such as flame holders 78 or elements of the premixer 60 suchas venturi exit 72 promotes a relatively "clean" combustion zone, thatis, devoid of objects which could fragment or separate from connectionsand be carried downstream to cause damage or catastrophic failure in theturbine. Thus, the extremely clean and simplified construction of thecombustor of the present invention is a further distinct advantage andbenefit.

As best seen in FIG. 1B, mixing venturi 68 is disposed such that venturiaxis 74 is oriented substantially tangentially with respect to housingaxis 42 such that the incoming fuel/air mixture is caused to swirl aboutaxis 42 within the combustion zone 54. It has been found using thepreferred premixer construction to be described in more detailhenceforth that combustion chamber 52 can be adequately fed by usingonly a single fuel/air premixer fed by a single fuel nozzle. However,the present invention contemplates the possible use of multiple fuel/airpremixers, particularly for situations wherein the radial "thickness" ofcombustion chamber 52 is small relative to the outer radius thereof, asmeasured from axis 42.

Advantageously, and in accordance with the present invention, thecombustor system preferably includes an ignitor disposed on thecylindrical liner housing at a location adjacent the intersection of theflow axis of the venturi. As embodied herein, and with continuedreference to FIG. 1B, ignitor 79 is positioned near the intersection offlow axis 74 and housing 40, and extends at most only a short distanceinto combustion zone 54. Ignitor 79 is thus ideally positioned tointercept the fuel/air mixture emanating from premixer 60 to initiatecombustion. Once started, the swirling hot combustion gases in zone 54will provide auto ignition of the fuel/air mixture and ignitor 79, whichmay be electrical, will normally be shut off.

Further in accordance with the present invention, compressed air conduitmeans are provided interconnecting the compressor exit and the fuel/airpremixer for delivering a portion of the compressed air flow to thepremixer compressed air receiving means and for delivering essentiallythe remaining portion of the compressed air flow to the aperture meansfor providing dilution air to the dilution zone. As embodied herein, andwith continued reference to FIG. 1A, compressed air conduit meansdesignated generally by the numeral 80 includes generally annularpassageway 82 disposed between pressure housing 14 and housing 40.Passageway 82 extends between compressed air receiving plenum 30 and aring-shaped plenum 84 and is formed as part of pressure vessel 14adjacent the turbine exhaust section. Fuel/air premixer housing assembly62 is connected to receive compressed air from plenum 84 for eventualcommunication to the venturi inlet 70 as explained previously. Plenum 84is shown having a circular cross section but other shapes,configurations and locations are possible and are considered within thescope of the present invention.

As can be appreciated from the schematic in FIG. 1A, passageway 82 isconfigured such that the compressed air flowing therein provides coolingfor housing 40, particularly housing portion 86 immediately surroundingthe combustion zone 54 where the highest combustion temperatures areexpected. Portion 86 of housing 40 is constructed for convection coolingonly, with no film-cooling necessary. That is, in portion 86 of housing40, the housing acts to seal off the compressed air flowing inpassageway 82 from the fuel/air mixture being combusted in combustionzone 54. This construction provides for control of the fuel/air ratio ofthe mixture in combustion zone 54 and permits operation as a "singlestage combustor" with a desired lean fuel/air ratio. Such an operationcan yield low levels of NO_(x) and unburned fuel and fuel by-productlevels. As will be discussed henceforth, the particular construction ofthe combustor system of the present invention permits extraordinarilylow levels of NO_(x) in comparison with other state of the artcombustion systems.

Passageway 82 essentially envelopes combustion chamber 52 to provideconvection cooling and also to supply compressed air to dilution ports58a and 58b. Passageway 82 also may include a channel 82a for channelingcompressed air flow for cooling the portion of the pressure vessel 14adjacent turbine 20, as is shown in FIG. 1A. Turbine inlet guide vanes34 may be film cooled inlet guide vanes and may be fed from passageway82 or 82a. Also, compressed air conduit means 80 can include a separatepassageway 88 interconnecting the compressed air receiving plenum 30 andair-blast fuel nozzle 64 when such a nozzle is used, particularly withliquid fuel operation.

As would be understood from the foregoing discussion in conjunction withFIG. 1A, compressed air conduit means 80 acts to channel a portion ofthe compressed air flow to the fuel/air premixer 60 and to channelessentially the remaining portion of the compressed air flow to thedilution ports 58a and 58b. The compressed air flow not channeled toeither the fuel/air premixer or the dilution ports, namely the air usedto cool the inlet guide vanes 34, is very small and in any event doesnot disturb the fuel/air ratio in the combustion zone but merely resultsin a small further dilution of the exhaust gases prior to entry intoturbine 20.

Further in accordance with one aspect of the present invention, valvemeans are disposed in the compressed air flow path for determining thecompressed air flow rate to the premixer. The compressed air valve meansis especially important where the speed of the compressor, and thus thevolumetric flow rate of compressed air, is essentially independent ofthe fuel flow rate, such as the application depicted in FIG. 1A. Asembodied herein and with continued reference to FIG. 1A, valve 90 ispositioned in fuel/air premixer housing assembly 62 for determining therate of compressed air flow from plenum 84 to venturi inlet 70. Valve 90is continuously adjustable, and a suitable construction of valve 90 willbe discussed in more detail hereinafter in relation to the descriptionof one preferred construction of the fuel/air premixer of the presentinvention. When the valve opening changes, the pressure drop over thepremixer changes, resulting in an increase or decrease of air mass flowto the dilution zone. Thus, this variation and dividing of the air flowhappen outside the combustor proper.

FIG. 2 discloses combustor system 110 having an alternate configurationfor the compressed air conduit means. Components having the same orsimilar function relative to the embodiment in FIGS. 1A, 1B are giventhe same numeral but with a "100" base. In the compressed air conduitmeans designated generally as 180 in FIG. 2, a distribution conduit 181is provided between compressed air collection plenum 130 and annularpassageway 182 surrounding housing 140, and fuel/air premixer housingassembly 162 is directly connected to distribution conduit 181 upstreamof passageway 182. Valve 190 is disposed at the connection betweenfuel/air premixer housing assembly 162 and distribution conduit 181 topositively divide the air flow into a first portion flowing to fuel/airpremixer 160 and the remainder to passageway 182 via distributionconduit portion 181a. As compared with the embodiment in FIG. 1A, wheresubstantially all of the compressed air portion flowing to the premixerwas first used to cool at least a part of liner housing portion 86defining combustion chamber 52, none of the compressed air portionflowing to fuel/air premixer 160 is used to cool portions 186 of housing140 defining combustion zone 152. However, the FIG. 2 embodiment doesallow for the direct control of the compressed air fractions flowing tothe fuel/air premixer versus the compressed air flow fraction flowing tothe dilution ports 158a and 158b. The configuration shown in FIG. 1A maybe preferred nonetheless because of an ease of construction in assemblyof the various components, principally the fuel/air premixer wherein thevalve can be integrated directly with the fuel/air premixer housing, aswill be discussed in more detail henceforth.

Further in accordance with the present invention, fuel conduit means isprovided interconnecting the fuel supply and the premixer fuel receivingmeans, the fuel conduit means together with the premixer fuel receivingmeans establishing a flow path for all the fuel to the premixer. Fuelvalve means is disposed in the fuel flow path for determining the fuelflow rate therein. As embodied herein, and with reference again to FIG.1A, fuel line 66 interconnects fuel source 32 with fuel nozzle 64. Fuelvalve 92 is disposed in fuel line 66 immediately upstream of fuel nozzle64, which is depicted as being an "air-blast" type fuel nozzleparticularly suitable for use with liquid fuels, as stated previously.

Still further in accordance with the present invention, the combustorsystem includes controller means operatively connected both to thecompressed air valve means and the fuel valve means for essentiallycontrolling the respective flow rates of the compressed air portion andthe fuel delivered to the premixer to provide a preselected leanfuel/air ratio mixture through the inlet port to the combustion zone. Asembodied herein and as depicted schematically in FIG. 1A, controller 94which can be either mechanical or electric (e.g., a microprocessor) isinterconnected with compressed air valve 90 to essentially control theflow rate of the compressed air flowing directly to venturi inlet 70.While a small portion (typically 5% or less), of the total compressedair flowing to fuel/air premixer 60 can travel through conduit 88 whenan "air-blast" nozzle is utilized, the control provided by valve 90 ofthe remaining 95+% of the compressed air flow is expected to achieveadequate overall fuel/air ratio control. Moreover, for situationsutilizing gaseous fuel, such as natural gas as provided in the Exampleto be discussed hereinafter, conduit 88 could be eliminated such thatall of the compressed air flow to the fuel/air premixer will be underthe control of the compressed air flow valve.

Also as depicted in FIG. 1A, controller 94 is operatively connected tofuel valve 92 to meter the fuel flow to fuel nozzle 64. As one skilledin the art would appreciate, controller 94 can act to control both thefuel flow and the compressed air flow to fuel/air premixer 60 to achievea single preselected fuel/air ratio mixture over the entire operatingrange of the gas turbine engine module so that the mass flow of thecombustible mixture would change as a function of the load. Or,alternatively, controller 94 can be configured to provide a sequence ofpreselected fuel/air ratio mixtures as a function of load. One skilledin the art would be able to select and adapt a suitable controller for aparticular application based on the present disclosure and the generalknowledge in the art.

In operation, and with reference to FIGS. 1A and 1B, compressed air fromcompressed air receiving means 30 is channeled via passageway/envelope82 over the outside surface of housing 40 for cooling housing 40, andparticularly portions 86 which surround combustion zone 54. A portion ofthe compressed air flowing in passageway 82 is admitted to plenum 84 andthen flows to fuel/air premixer 60 via the interconnection betweenfuel/air premixer housing assembly 62 and 84 as controlled by compressedair valve 90 via controller 94. In venturi 68, the compressed airportion is mixed with the fuel from fuel nozzle 64, possibly with asmall additional portion of compressed air if nozzle 64 is a "air-blast"type nozzle, and is injected along the venturi axis 74 through inletport 43 and into combustion zone 54 of combustion chamber 52.

As shown in FIG. 1B, swirling flow and combustion is provided incombustion zone 54 by locating venturi axis 74 tangentially with respectto axis 42 of the housing. The direction of orientation of the venturiaxis 74 is chosen to give a specific angular direction (clockwise orcounterclockwise) with respect to the direction of rotation of theturbine in order to provide some aerodynamic unloading of the inletguide vanes. For the configuration depicted in FIGS. 1A and 1B where thefuel/air mixture is admitted to achieve a clockwise swirling combustionin combustion zone 54 as viewed in the direction AA, the direction ofrotation of turbine 20 also would be in the clockwise direction. Aftercombustion of the fuel/air mixture in zone 54, the hot exhaust gasespass to dilution zone 56 where dilution air from dilution ports 58a, 58breduce the average temperature of the exhaust before it is ducted viachannel 50 past inlet guide vanes 34 to turbine 20 for work-producingexpansion.

The control of combustion afforded by combustion system 10 in accordancewith the present invention through the complete mixing of the fuel andair outside the combustion chamber in the fuel/air premixer, includingcomplete vaporization of the fuel if liquid fuel is used, together withthe control of the fuel/air ratio of the mixture delivered to thecombustion chamber allows for significant reductions in NO_(x) levelsand the levels of unburned fuel and fuel by-products, as mentionedearlier. Furthermore, the utilization of essentially the total amount ofcompressed air flow to either combust the fuel or to dilute the exhaustgases upstream of the turbine provides considerable reduction of peakcombustor temperatures resulting in longer life for combustor linerscompared to conventional combustor designs.

As previously mentioned, the preferred fuel/air premixer of the presentinvention includes a compressed air receiving means, a venturi having aninlet operatively connected to the compressed air receiving means withair flow smoothing means, a fuel receiving means including a nozzle withan exit positioned to deliver a spray of fuel into the venturi inletsubstantially along the venturi axis, and valve means associated withthe compressed air receiving means for determining the compressed airflow rate to the venturi inlet. As embodied herein and with reference toFIG. 3A, fuel/air premixer 260 includes air receiving means in the formof housing assembly 262. Components having a like or similar function tothose disclosed in the embodiments of FIGS. 1A and 1B will be designatedby the same numeral but with a "200" base. Housing assembly 262, inturn, includes housing 300 and housing support 302 for mounting housing300 on pressure vessel 214 of gas turbine engine module 212. Housingsupport 302 is hollow and, in addition to supporting housing 300 and thecomponents contained therein, acts to channel compressed air from plenum284 to housing 300. In the construction shown in FIG. 3A, cooling shroudmember 303 is positioned between combustion chamber liner housing 240and pressure vessel 214 for establishing the flow path 282, at least inthe vicinity of portions 286 of housing 240 that define the boundary ofthe combustion zone 254. Shroud member 303 also defines with pressurevessel 214, plenum 284 for collecting the compressed air portion foreventual transmission to housing 300 via housing support 302.

With continued reference to FIG. 3A, fuel/air premixer housing 300 isdivided into upstream and downstream compartments 304, 306 respectivelyby divider plate 308. Aperture 310 is provided in divider plate 308, anda butterfly-type valve plate 290 is mounted for rotation in aperture310. In the FIG. 3A embodiment, the orientation of valve plate 290 inaperture 310 is controlled through control arm 312 (see FIG. 3B) toprovide a selective degree of obstruction and, hence, pressure drop. Inthe orientation of valve plate 290 shown in FIGS. 3B and 3C, a minimumamount of obstruction occurs with valve plate 290 being orientedperpendicular to the divider plate 308, corresponding to a "zero"setting of the angular calibration plate 314 shown in FIG. 3C. Aposition of control rod 312 corresponding to either "9" position onindicator 314 would result in the greatest amount of obstruction andpressure drop in the compressed air portion flowing through aperture310. As one skilled in the art would realize, the degree of obstructionand thus control of the compressed air flow between upstream compartment304 and downstream compartment 306 could be varied by changing theangular orientation of control rod 312 between the "zero" and "9"positions, thereby controlling the compressed air flow rate to thebalance of the fuel/air premixer 260 which will now be described in moredetail.

Divider plate 308 includes an additional aperture 316 in which ismounted inlet 270 of venturi 268. Venturi inlet 270 is configured andmounted to divider plate 308 such that a smooth transition existsbetween the upper planar surface of divider plate 308 and the innersurface of venturi inlet 270. Venturi 268 extends through upstreamhousing compartment 304, housing support 302, past pressure vessel 214,combustion chamber liner 303, and connects to housing 240 at thelocation of inlet port 243. As described previously in relation to theembodiment depicted in FIG. 1A, the venturi axis 274 which correspondsgenerally to the flow direction of the fuel/air mixture in venturi 268is oriented to provide a substantially tangential admission directionwith respect to the axis (not shown) of annular combustion chamberhousing 240.

With continued reference to FIG. 3A, fuel nozzle 264 is mounted indownstream compartment 306 with the fuel nozzle exit 318 positioned todeliver a spray of fuel into venturi inlet 270 along venturi axis 274.Fuel nozzle 264 is of the "swirling" spray type which utilizes ports 320and swirl vanes 322 to channel some of the compressed air swirl the fuelentering through fuel port 324 before releasing the fuel spray throughexit 318. Also shown in FIG. 3A is perforated flow-smoothing element 326positioned in the downstream compartment 306 and surrounding fuel nozzleexit 318 and venturi inlet 270, to avoid uneven velocities andseparation in the venturi which otherwise could result in "flameholding" in the venturi. While a small pressure drop is introduced byits incorporation, the perforated element 326 has been found to provideincreased stability for the compressed air flow from downstreamcompartment 306 past the fuel nozzle 264 and into venturi inlet 270,without any separation at the lip of venturi inlet 270.

FIG. 4 shows a contemplated commercial variation of the preferredfuel/air premixer depicted in FIGS. 3A-3C, which variation is designatedgenerally by the numeral 360. Components having the same or similarfunction to those described in relation to the FIGS. 1A, 1B embodimentare given the same numerals but with "300" base. Fuel/air premixer 360includes a venturi 368 which has inlet 370 which extends slightly abovethe surface of divider plate 408. Also, fuel nozzle exit 418 extends adistance into venturi inlet 370. One skilled in the art would realizethat the optimum performance of the fuel nozzle 364 in conjunction withthe venturi 368 (as well as nozzle 264 and venturi 268 in the variationshown in FIGS. 3A-3C) may vary from application to application and thatthe positioning of fuel nozzle exit 418 along the venturi axis 374 inthe vicinity of venturi inlet 370 may be adjusted to determine theoptimum position. However, it is anticipated that perforated screenelement 426 would provide flow stability for the FIG. 4 embodiment aswell. Finally, the FIG. 4 embodiment incorporates contemplatedrefinements in the construction of the fuel/air premixer compared to theconstruction shown in FIG. 3A, such as the use of integral, bell-shapedhousing 400.

As mentioned previously, certain aspects of the present inventionadvantageously can be adopted for applications such as gas turbine gasgenerator modules used in conjunction with free power turbines or freejet propulsion units, which gas generator modules may not require theuse of a compressed air flow valve and associated controller functions,in contrast to the previously discussed embodiments depicted in FIGS. 1Aand 2. FIG. 5A depicts schematically such an engine system constructedin accordance with the present invention and designated generally by thenumeral 500. Engine 500 comprises gas turbine gas generator module 512,including combustor system 510 to be discussed in more detailhereinafter and free power turbine module 513. Free turbine module 513includes free turbine 513a which is depicted as an axial turbine, butcould be pure radial or mixed axial-radial as the application mayrequire. In comparison with the FIG. 1A engine system embodiment wherepower was extracted from gearing 24 connected to shaft 16, power istaken from the engine system 500 in the FIG. 5A embodiment via gearingassociated with free turbine shaft 513b. Although shown coaxial withaxis 518 of the gas generator module, rotational axis 513c of free powerturbine 513 could be angularly displaced to meet the requirements of theoverall system 500.

In the subsequent discussion, like components relative to the embodimentin FIG. 1A will be given the identical numeral but with a "500" prefix.

In accordance with the present invention gas turbine gas generatormodule 512 includes a mechanically independent spool, namely centrifugalcompressor 522 and radial turbine 520 mounted for dependent rotation onshaft 516, inside pressure housing 514. Thus, shaft 516 can rotateindependently of free turbine shaft 513b although gas generator 512 andfree turbine module 513 are interconnected in the gas flow cycle. Module512 also includes combustor system 510 with combustor liner housing 540which is contained within pressure housing 514 and which receivespremixed air/fuel from external premixer 560 through inlet port 543along venturi axis 574. Venturi axis 574 is oriented tangentially withrespect to axis 542 of annular combustor liner housing 540 to provideefficient, swirling combustion and also to partially unload inlet guidevanes 534, as discussed previously in relation to the FIG. 1Aembodiment. See FIG. 5B.

FIG. 5B also depicts the presently preferred position of ignitor 579,namely on liner housing 540 adjacent the intersection of venturi axis574. While it may eventually be possible to locate the ignitor in arelatively cooler environment, such as in the premixer, and therebyprolong ignitor life and further decrease the number of penetrations inliner housing 540, the location depicted in FIG. 5B is expected toensure light-off because of the low velocities of the fuel/air mixturein the annular chamber.

In the embodiment depicted in FIGS. 5A and 5B, housing liner 540 andpressure housing 514 cooperate to form passages for the compressed airflow from compressor plenum 530. Also included in this engine embodimentis annular cooling shroud 583 disposed between, and radially spaced fromboth, housing liner 540 and the circumferentially adjacent portion ofpressure housing 514. As can be appreciated from the figures, coolingshroud 583 and housing liner 540 cooperate to form part of thepassageway 582 for convectively cooling the combustor chamber defined byliner 540 while cooling shroud 583 and pressure housing 514 cooperate toform annular plenum 584 to collect the portion of the compressed airflow to be channeled to premixer 560 for mixing with the fuel. In theFIG. 5A embodiment, as in the embodiment shown in FIG. 1A, a portion ofthe compressed air is taken from the passageway leading from thecompressor exit after providing convective cooling and is then channeledto the premixer for mixing with fuel, but the FIG. 5A arrangement can bemade more structurally compact than the ring-shaped plenum 84 in FIG.1A. Furthermore, cooling shroud 583 provides radiation shielding of theadjacent parts of pressure housing 514 from the relatively hot linerhousing 540, allowing the use of less expensive materials and increasingthe service life of the pressure housing.

The balance of the compressed air flow in passageway 582 is channeledthrough dilution apertures 558b. There are no dilution portscorresponding to the ports 58a in the FIG. 1A embodiment, but dilutionports 558b include two separate circumferential port sets 558b₁ and558b₂. Divider 559 and the sizing of ports 558b₁ and 558b₂ causesdilution air flowing through ports 558b₂ to first flow throughpassageway 582a past turbine shroud 557. One skilled in the art would beable to perform the required sizing analysis to provide adequatedistribution of the dilution air to achieve desired turbine shroudcooling. The elimination of film cooling provides for control over thefuel/air ratio in the combustion zone 554 and is one of the highlysignificant benefits and advantages of the present invention, asexplained previously.

FIG. 5A also shows (in dotted line) conduit 588 leading from compressorexit planum 530 to premixer 560 in the event "air-blast" type liquidfuel nozzle is utilized, for reasons explained previously. Althoughshown penetrating compressor plenum-exit 530 axially inclined in FIG. 5Afor clarity, the inlet to conduit 588 would be tangential and in theaxial plane of the compressor exit to capture the total dynamic head.One skilled in the art would be able to design an appropriate inletconfiguration given the present description.

Aside from the small amount of compressed air that may be required tooperate an air blast-type liquid fuel nozzle and, possibly, for inletguide vane cooling, all of the compressed air is used to convectivelycool at least part of liner housing 540 before being used for mixingwith the fuel or for dilution. This construction optimizes theconvective cooling capacity of the compressed air inventory. Althoughnot shown, the present invention is also intended to include a gasgenerator variation corresponding to the FIG. 2 embodiment where thecompressed air flow portion used for mixing with the fuel is not firstused for convective cooling. The simplified construction of such asystem might outweigh the reduction in cooling capacity and therefore bedesired for certain applications.

As depicted in FIG. 5A, air is channelled from passageway 582 throughannular plenum 584 for mixing directly with the fuel in premixer 560.FIG. 5A depicts compressed air valve 590 by broken lines to indicatethat the valve is optional. It may be used for "fine tuning" thefuel/air ratio during operation, it may be preset to a fixed opening foroperation, or it may be eliminated entirely, for the following reason.In engine system 510, the speed of compressor 522 and thus thecompressed air flow rate is essentially proportional to the fuel flowover the operating range. Hence, gross control of the fuel/air ratio toa preselected lean value can be achieved automatically. The function ofcontroller 594 acting to control fuel flow to fuel nozzle 564 fromsource 532 through fuel valve 592 thus becomes similar to that of aconventional throttle responsive to power demands.

While premixer 560 channels all the fuel/air mixture to combustion zone554 required over the intended operating range of engine system 510, anauxiliary fuel supply system such as system 596 depicted in FIG. 5B maybe used to provide a richer mixture for start-up and idle conditions.System 596 includes a conventional fuel spray nozzle 597 fed from fuelsource 532 (see FIG. 5A), and the auxiliary fuel flow rate can becontrolled by controller 594 through valve 598. In the disclosedembodiment, spray nozzle 597 is positioned to penetrate liner housing540 adjacent venturi outlet 572 and disposed radially. However, nozzle597 can be positioned in an opposed tangential orientation relative toventuri 568 (not shown) to enhance mixing with the fuel/air mixtureentering through venturi 568. Other positions, constructions andorientations of spray nozzle 597 are, of course, possible and areconsidered to fall within the general teachings herein.

FIG. 6 is a schematic of an alternative "valve-less" premixer designwhich could be used in engine system 510, and which is designatedgenerally by the numeral 660. Premixer 660 includes housing 662, fuelnozzle 663 which is of the type having peripheral swirl vanes 665, andventuri 668 oriented with venturi axis 674 tangential to the combustoraxis (not shown). Also, perforated flow-smoothing member 667 surroundsnozzle 664 and the entrance to venturi 668, for reasons explainedpreviously in relation to the corresponding components in the "valved"embodiment in FIG. 3A. Premixer 660 additionally includes heating meanssuch as electric resistance heater jacket 669 surrounding the throatarea of venturi 668 and operatively connected to a power source (notshown) via electrical leads 671. During start up and using liquid fuels,a film of fuel tends to collect on the venturi inner surface. Heaterjacket 669 augments vaporization of this fuel film and thus promotes theoverall mixing of the fuel and air in the premixer. During operation,the temperature of the compressed air portion flowing past the outersurface of venturi 668 from plenum 684 may provide sufficient heat forvaporizing a liquid film, or prevent the formation of a liquid fuel filmaltogether, thereby dispensing with the need for continued activation ofheating jacket 669.

FIG. 7 schematically depicts yet another engine embodiment that mayadvantageously utilize the combustor of the present invention, namely, agas turbine engine system such as described in my previous patent U.S.Pat. No. 5,081,832, the disclosure of which is hereby incorporated byreference. In the FIG. 7 embodiment, engine system 700 includes highpressure spool 711 and mechanically independent low pressure spool 709.Low pressure spool 709 includes low pressure compressor 701 which isdriven through shaft 702 by low pressure turbine 703. The compressed airexiting low pressure compressor 701 flows through diffuses 704 andenters high pressure compressor 722 for further compression. Ascomponents of high pressure spool 711 high pressure compressor 722 isdriven by high pressure turbine 720 via shaft 716. Gases exhausted fromhigh pressure turbine 720 are diffused in diffuser 705 and then expandedin low pressure turbine 703. For reasons explained more fully in U.S.Pat. No. 5,081,832, net power is taken from engine system 700 viagearing 724 connected to shaft 716 of high pressure spool 711. Lowpressure spool 709 is used principally to supply pre-compressed air tohigh pressure spool 711 and possibly to drive engine support systems(e.g., lubrication).

As seen in FIG. 7, engine system 700 includes combustor system 710 toprovide hot combustion gases to high pressure turbine 720 by combustingfuel with a portion of the compressed air from high pressure compressor722. Importantly, combustor system 710 uses external premixer 760 whichincludes fuel nozzle 764 (which may be an "air-blast" type receivingcompressed air directly from compressor 722 via conduit 788 with atangential inlet-shown dotted) and venturi 768 to supply fully premixedfuel/air tangentially to annular combustion zone 754 defined by linerhousing 740. Cooling shroud 783 and liner housing 740 cooperate todefine part of convective cooling passageway 782, while cooling shroud783 and the circumferentially adjacent portion of pressure housing 714cooperate to form annular plenum 784 to channel a portion of thecompressed air to premixer 760. The balance of the compressed air flowis used for additional convective cooling and finally dilution, using aconfiguration and construction similar to that shown in FIG. 5A.

However, the engine system configuration shown in FIG. 7 is intended forproducing power at essentially constant high pressure spool shaft speed.Like the FIG. 1A embodiment, the total compressed air flow rate will notautomatically adjust to a changed fuel flow in the manner of gasgenerator module 512 in the FIG. 5A embodiment. As a result, combustorsystem 710 specifically includes compressed air valve 790 integratedwith premixer 760 and under the control of controller 794, which alsocontrols fuel valve 792, to achieve a preselected lean fuel/air ratio.It is understood that, although not shown, the FIG. 7 embodiment couldinclude features described in relation to the other embodiments,including a liner-mounted ignitor, auxiliary fuel spray system, stageddilution ports, etc.

EXAMPLE

In order to assess the performance of the annular combustor system ofthe present invention, an annular combustor having the fuel/air premixeras shown in FIGS. 3A-3C was atmospherically tested using an externalsource of air and a gaseous fuel (natural gas). Table 1 presents valuesof the important dimensions of the apparatus used in the test.

                  TABLE 1                                                         ______________________________________                                        Volume of combustion chamber                                                                        (12.3 × 10.sup.-3 m.sup.3)                        Outer diameter of combustion zone                                                                   (0.346 m)                                               Inner diameter of combustion zone                                                                   (0.200 m)                                               Radial distance from the housing                                                                    (0.124 m)                                               axis to the venturi axis                                                      Diameter of the venturi                                                       Throat                (45 mm)                                                 Exit                  (75 mm)                                                 Perforated element hole diameter                                                                    (.O slashed.3.75 × 5 mm)                          and pitch                                                                     ______________________________________                                    

Tests were done at flow conditions corresponding to idle and full load.Flow rates to achieve a preselected fuel/air ratio were set by manuallysetting compressed air valve 290 and the fuel valve (not shown) ratherthan by a controller element although a controller element could havebeen used. Table 2 presents the fuel and compressed air flow rates andother important parameters as well as the measured NO_(x) levels andapproximate CO emission levels for the tests.

                  TABLE 2                                                         ______________________________________                                                             IDLE    FULL LOAD                                        ______________________________________                                        BTU rating of natural gas (MJ/kg)                                                                  38.02   38.02                                            Fuel flow rate (g/s) 2.45    3.12                                             Total air flow rate: (g/s)                                                                         183     160                                              Fuel/Air Ratio       0.033   0.033                                            Compressed air inlet temperature (°C.)                                                      376     430                                              Total pressure loss (percent):                                                                     5       3                                                Total air factor:    2.3     2.3                                              Pattern factor (percent):                                                                          11      8                                                NO.sub.x  (ppm) at 15% O.sub.2 :                                                                   5       3                                                ______________________________________                                    

The above indicates remarkably low NO_(x) emission levels which, even ifscaled for high pressure operation, still would be well below the valuesconsidered representative of state of the art gas turbine engine modulecombustor systems using premixers. See G. Leonard et al., "Developmentof Aero Derivative Gas Turbine DLE Combustion System", Diesel and GasTurbine Worldwide, May, 1993, pp. 22 and 24.

With the above detailed description of the annular combustor system andfuel/air premixer apparatus and method of operating same of the presentinvention, those skilled in the art would appreciate that modificationsmay be made to the invention without departing from its spirit.Therefore, it is not intended that the scope of the invention be limitedto the specific embodiments illustrated and described above. Rather, itis intended that the scope of this invention be determined by theappended claims and their equivalents.

What is claimed is:
 1. A combustor system for operation with a source ofcompressed air and a source of fuel, the combustor system comprising:acylindrical housing defining a single stage combustion chamber, saidhousing having an axis; at least one fuel/air premixer disposed outsidesaid cylindrical housing; a first conduit interconnecting the source ofcompressed air and said premixer; a second conduit interconnecting thefuel supply and said premixer; and a fuel valve for determining the fuelflow rate to said premixer through said second conduit, wherein saidpremixer includes a venturi having an inlet, an outlet, and a flow axis,said venturi inlet being disposed to receive the compressed air andfuel, and said outlet being flow connected to said cylindrical housingto deliver a substantially mixed fuel/air mixture to said combustionchamber, said venturi being disposed with said flow axis being alignedin a substantially tangential direction with respect to said housingaxis, and wherein said premixer further includes an air valveoperatively connected to control the compressed air flow to said venturiinlet.
 2. The combustor system as in claim 1, wherein said combustionchamber includes a combustion zone, and wherein said first conduitincludes at least one passageway for convectively cooling the part ofsaid cylindrical housing defining said combustion zone with compressedair, the compressed air flowing in said passageway being sealed off fromsaid combustion zone by said defining housing part.
 3. The combustorsystem as in claim 2, wherein said first conduit also provides forconvectively cooling said defining housing part with the compressed airflowing to said premixer.
 4. The combustor system as in claim 1, furtherincluding an ignitor positioned on said cylindrical liner housingadjacent the intersection of said venturi flow axis.
 5. The combustorsystem as in claim 1,wherein said premixer includes at least one fuelnozzle positioned to direct a fuel stream into said venturi inletsubstantially along said venturi axis; and wherein said premixer fuelnozzle is an air blast nozzle to provide initial premixing of the fueland air, said first conduit including a conduit member directlyinterconnecting the source of compressed air and said air blast nozzle.6. The combustor system as in claim 5, wherein the fuel is liquid fuel,and wherein said air blast nozzle also provides atomization of theliquid fuel prior to entry into said venturi inlet.
 7. The combustorsystem as in claim 1,wherein said premixer includes a housing definingin part an inlet flow path for the compressed air into said venturiinlet; and wherein a flow-smoothing member is positioned in said inletflow path.
 8. The combustor system as in claim 1, wherein said venturioutlet terminates substantially flush with said housing.
 9. Thecombustor system as in claim 1, having only said one fuel/air premixerfor providing the fuel/air mixture to said combustion chamber.
 10. Thecombustor system as in claim 1, further including means for heating saidventuri for augmenting the vaporization of liquid fuels.
 11. A gasturbine gas generator module comprising the combustor system of claim 1wherein said cylindrical housing includes an exhaust gas port andfurther comprising a spool having an air compressor with an exit, aturbine, and a shaft assembly interconnecting said turbine and said aircompressor, the module still further comprising an exhaust gas flow pathfrom said exhaust port and through said turbine, wherein said firstconduit is operatively connected to said air compressor exit, andwherein essentially all of the compressed air from said air compressornot flowing to said premixer is admitted to said exhaust gas flow pathupstream of said turbine.
 12. The gas turbine gas generator module as inclaim 11, further including, inlet guide vanes positioned in saidexhaust gas flow path upstream of said turbine, and wherein saidsubstantially tangential alignment direction of said venturi axis isselected relative to the direction of rotation of said turbine toaerodynamically partly unload said inlet guide vanes.
 13. A gas turbineengine including the gas generator module of claim 11, furthercomprising power-producing means positioned in said exhaust gas flowpath downstream from said turbine.
 14. The gas turbine engine as inclaim 13, wherein said power-producing means is a free-power turbine.15. A gas turbine engine including the gas generator module of claim 11,wherein said spool is a high pressure spool having a high pressure aircompressor and a high pressure turbine, the engine further including alow pressure spool with a low pressure air compressor operativelyconnected to deliver pre-compressed air to said high pressurecompressor, and a low pressure turbine operatively connected to receiveand further expand exhaust gases from said high pressure turbine, and ashaft interconnecting said low pressure compressor and said low pressureturbine, said low pressure spool being mechanically independent of saidhigh pressure spool.
 16. A power generating unit comprising the gasturbine engine of claim 15 and a gearing assembly operatively connectedto said high pressure spool for delivering power.
 17. The combustorsystem as in claim 1, further comprising a cooling shroud partiallysurrounding and spaced from said cylindrical housing, and a pressurehousing surrounding and spaced from said cylindrical housing and saidcooling shroud, said cooling shroud and said pressure housing definingat least a part of a compressed air flow path from the compressed airsource to said premixer.
 18. The combustor system as in claim 1, furthercomprising a cooling shroud partially surrounding and spaced from saidcylindrical housing, and a pressure housing surrounding and spaced fromsaid cylindrical housing and said cooling shroud, said cooling shroudand said cylindrical housing defining at least a part of a passagewayfor convectively cooling at least the part of said cylindrical housingdefining a combustion zone in said combustion chamber, said passagewaybeing defined in part by said first conduit, the compressed air flowingin said passageway being sealed off from said combustion zone by saiddefining housing part, and said cooling shroud and said pressure housingdefining at least a part of a compressed air flow path from thecompressed air source to said premixer.
 19. The combustor system as inclaim 2, further comprising a cooling shroud partially surrounding andspaced from said cylindrical housing, and a pressure housing surroundingand spaced from said cylindrical housing and said cooling shroud, saidcooling shroud and said cylindrical housing defining at least a part ofsaid passageway, and said cooling shroud and said pressure housingdefining at least a part of a compressed air flow path from thecompressed air source to said premixer.